Depletion and rising prices of oil resources have been concerned about in recent years. Also, a shift from the oil resources that constitute one of the factors contributing to global warming to new energy resources has been studied. One of the new energy sources is solar thermal power generation which concentrates and uses solar light as energy.
A beam-down type solar light concentrating system is known as one of the solar light concentrating systems. The beam-down type solar light concentrating system is that which is configured to reflect the solar light to an upper part of a center part of the system by using heliostats serving as reflecting mirrors and to concentrate the reflected light onto a receiver (a heat receiving unit) installed at its lower part by using a large reflecting mirror called a center reflector (see Patent Document 1, for example).
This receiver may be formed of pipes or the like in the inside of which a heating medium (such as fused salt) circulates. In this case, the heating medium having received thermal energy from the solar light by way of the receiver is sent to a power generating unit. The power generating unit includes a power generator or the like, which is configured to generate steam by use of the heat in the heating medium, and to generate power by supplying the steam to a steam turbine.
An invention in which a heat collection receiver is formed in a pot shape (a cavity shape) is disclosed as a method of improving the efficiency of power generation utilizing this solar light concentrating system (see Patent Document 2). The invention recited in Patent Document 2 is configured to cause the solar light that enters a pot to be reflected several times so as to transfer heat efficiently to a heating medium and to retain the heat inside. This configuration significantly improves the efficiency of solar power generation.
However, because of its structure in which a single pipe is formed in the pot shape, the pot-shaped receiver recited in Cited Document 2 has problems of a great difficulty in manufacture and an increase in the manufacturing cost. Moreover, since the size of the receiver becomes huge, the receiver of an integrated type is not easy to transport, and it is a difficult task to construct the receiver on site as well.
In addition, the entire receiver needs to be replaced in case of a failure of the receiver such as pipe breakage. In this respect, in the case of constructing a beam-down type solar light concentrating system of, for example, 400 to 500 meters in each side, the diameter of the pot-shaped receiver becomes as huge as about 8 to 10 m.
Moreover, regions inside the receiver exposed to the solar light are uneven. Thus, uneven temperature distribution leads to unevenness in the amount of thermal expansion of the pipe constituting the receiver. Accordingly, the receiver has problems of causing a gap, expansion in a small space, and resulting in a failure such as a crack on the pipe as the case may be.
Further, when the receiver causes fails due to pipe breakage or the like, the receiver needs to be entirely replaced because the receiver is formed of the single pipe. Hence, the replacement involves an extensive operation. In addition, there is a problem of costly preparation of a new receiver.
Furthermore, solar light concentrating systems often use fused salt such as sodium nitrate, which turns into a liquid phase in a range of 150° C. to 500° C., as the heating medium. In a case where the fused salt is used, the fused salt needs to be removed from the receiver and transferred to a heat retention tank on a cloudy day or at night. This is because the fused salt is solidified when cooled down, and causes a clog inside the pipe of the receiver. When the fused salt is recovered, it takes time to remove the fused salt from the receiver disclosed in Cited Document 2, which includes a spiral pipe. In particular, there is a problem that the receiver cannot handle an emergency when the fused salt needs to be recovered quickly.